In the production of welded pipes according to API standards, a normalization of the weld and heat affected zone (HAZ) is required. The weld of thick-walled tubes shows an hour-glass shaped HAZ caused by the "corner effect" in the weld Vee. The microstructure, particularly at the external and internal surfaces, is very coarsegrained. The aim for the heat treatment is to reestablish a homogeneous and finegrained microstructure in the HAZ. To achieve a fine-grained microstructure after the normalization, it is important to limit the temperature on the external surface of the pipe, which is closest to the coil, to avoid grain re-growth, and to reach high enough austenitisation temperature in a sufficiently wide zone at the inner surface. The width at the inner surface must cover the width of the HAZ, which has its maximums at the surfaces, and the positioning tolerance of the heat zone to the weld. Even if low frequency is used for the reheating of the weld zone, the heat penetration through the pipe wall is partly reliant on heat conduction. Time needed for temperature equalization through heat conduction increases proportionally to the square of the wall thickness. The preferred welding method is by induction, which requires higher minimum welding speed (minimum 10 - 12 m/min) than contact welding (minimum 5 - 6 m/min). One complication with in-line normalizing heat treatment of thick-walled pipes is that the heating requires considerable length in a line. This is due to the time needed for temperature equalization through the pipe wall and the required line speed. An important design work is to optimize the temperature distribution in the heat zone and establish the minimum heating length required. A 2D simulation, using coupled electromagnetic and transient thermal calculations in a cross-section of the weld as it passes through the different heating zones, is today an indispensable tool. It is important to have good representations of the very nonlinear and temperature dependant electrical and physical properties of the steel, as well as heat losses by convection and radiation from the surfaces and residual heat from the welding.
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